1. Field of the Invention
The present invention relates to a light emitting diode and more particularly to a gallium arsenide (GaAs) infrared-light emitting diode having a high luminescent efficiency.
2. Description of the Prior Art
A conventional GaAs infrared-light emitting diode having a high luminescent efficiency comprises an Si-doped n-type GaAs substrate, an Si-doped n-type GaAs layer formed on the substrate, an Si-doped p-type GaAs layer formed on the n-type GaAs layer to form a PN junction therebetween, and a pair of electrodes in ohmic contact with the substrate and the p-type GaAs layer.
Si atoms doped into the GaAs crystal give rise to a shallow acceptor level of 0.03eV and a deep acceptor level of 0.1eV. When the PN junction of such a GaAs infrared-light emitting diode as described above is forwardly biassed, the infrared radiation associated with the deep acceptor level takes place in the p-type GaAs layer.
A relatively high luminescent efficiency of 3 - 4 % can be obtained by taking the infrared radiation from the side of the n-type GaAs substrate whose infrared absorption coefficient is smaller than that of the p-type GaAs layer.
The reason why the GaAs infrared-light emitting diode having such a structure as described above is usually used, is as follows.
1. Since the energy of the infrared radiation associated with the deep acceptor level of 0.1eV is much smaller than the band-gap energy of GaAs, the amount of the absorption of the infrared light by the GaAs layer is relatively small so that a large luminescent output can be obtained.
2. Si atoms serve as amphoteric impurity to Ga and As. Therefore, if the temperature during liquid phase growth is high Si atoms are located in lattice points of Ga atoms to serve as donors, while at low temperature Si atoms are located in lattice points of As atoms to serve as acceptors. Accordingly, by utilizing this amphoteric property, both p-type and n-type layers can be formed in a single process of liquid phase growth so that the fabrication process can be facilitated and that a PN junction with excellent crystallization can be obtained. In other words, the luminescent efficiency can be improved.
A conventional GaAs light-emitting diode consisting only of Si-doped GaAs layers, however, has such drawbacks as follows, which degrade the luminescent efficiency.
1. Since Si atoms serve as amphoteric impurity in GaAs, the Si-doped GaAs crystal, no matter what type, p or n, formed through liquid phase growth becomes a compensated region. This is conjectured from the fact that the mobility of carriers in the resultant Si-doped GaAs crystal is smaller than that in an Sn-doped GaAs crystal or a Te-doped GaAs crystal, having the same carrier concentration as the Si-doped GaAs crystal, and is also manifest from the fact that the acceptor levels of 0.03eV and 0.1eV can be observed through the measurement of photoluminescence. It is therefore considered that the S-doped n-type GaAs layer serves an an absorber of the infrared light emitted from the Si-doped p-type GaAs layer. According to the conventional fashion, the emitted light is taken from the side of the n-type GaAs layer since the Si-doped n-type GaAs layer has a smaller absorption coefficient than the Si-doped p-type GaAs layer.
2. In the conventionally used Si-doped GaAs substrate, Si atoms as dopants near and in the surface of the substrate are sometimes oxidized to produce SiO.sub.2. The substance SiO.sub.2 is thermally stable and cannot be reduced to Si but remains as SiO.sub.2 under an ordinary condition required in the liquid phase growth of GaAs (at temperatures below 1000.degree. C in an atmosphere of H.sub.2). Consequently, the wettability of Ga solution with respect to the substrate becomes poor so that the grown crystal becomes uneven or that the uniformity of the epitaxially grown crystal is degraded due to SiO.sub.2 being contained in the crystal.
In order to eliminate the above drawbacks and therefore to improve the luminescent efficiency of the GaAs infrared-light emitting diode, it is necessary to substitute for the conventional Si-doped p-type GaAs layer a p-type Ga.sub.1.sub.-x Al.sub.x As layer which has wider band gaps than GaAs and which has a small absorption coefficient with respect to the light emitted from the GaAs, and to take out the light through the Ga.sub.1.sub.-x Al.sub.x As layer. A diode having such a structure as to embody this requirement has been proposed. In the fabrication process of such a diode, however, the Ga.sub.1.sub.-x Al.sub.x As layer must be formed on the GaAs layer through heterogenous epitaxial growth so that two steps of epitaxial growth are needed. Moreover, the manipulation of growing the three-element system is complicated, thus rendering the yield poor and rendering the completed diode expensive.